Hygienic articles such as disposable diapers, sanitary napkins, incontinence pads and sick bed sheets must have a high moisture absorption capacity to effectively retain eliminated body fluids for acceptable periods of time. Early hygienic articles of this type employed cellulose wadding, fluff cellulose or absorbent cotton as the absorbent materials. One problem with these materials is that their moisture-retaining capacity is relatively small compared to their volume. In order to improve the moisture-retaining capacity of hygienic articles made from these materials, the volume of such absorbent materials in the hygienic article must be increased. This produces a bulky product which is unacceptable in many hygienic articles, particularly sanitary napkins.
In an effort to reduce the volume and size of hygienic articles, and increase their absorbent capacity, one layer of such articles comprises a non-woven pad formed of cellulose fluff, wood pulp, textile fibers or other non-woven fibrous materials in which highly moisture-absorbent material is combined within the fiber structure of the pad. These highly moisture-absorbent materials are preferably water-insoluble absorbent polymers having a high capacity for absorbing water and body fluids, which are partially or wholly synthetic and are commercially available in fine-grain, particulate form. See, for example, U.S. Pat. Nos. 3,997,484; 3,661,815; 4,117,222; and, 3,936,441.
One system for forming the non-woven fibrous materials into a non-woven pad is disclosed in U.S. Pat. No. 3,984,272 In this system, a forming chamber having an inlet and outlet is connected by a feed conduit to a source of fibrous materials such as finely ground wood pulp. A perforated conveyor is movable through the forming chamber between its inlet and outlet, and structure is provided to draw a vacuum on one side of the perforated conveyor. The fibrous material introduced into the chamber is drawn onto the opposite side of the conveyor by the vacuum, and this fibrous material forms a non-woven pad on the conveyor which increases in thickness in a direction from the inlet of the chamber toward its outlet. The non-woven pad thus has a bottom surface resting atop the conveyor and a top surface which angles or slopes upwardly relative to the conveyor from the inlet of the chamber toward its outlet. A leveling or scarfing roller near the outlet of the forming chamber is operative to remove at least a portion of the fibrous material at the top of the non-woven pad to produce a non-woven pad of uniform thickness. The pad is then transmitted by the conveyor through the outlet of the forming chamber for subsequent operations to form the completed hygienic article.
Two methods have been employed to incorporate highly moisture-absorbent material into the non-woven pad formed within the forming chamber in systems of the type disclosed in U.S. Pat. No. 3,984,272 and described above. In one method, the highly moisture-absorbent material is injected into the same feed conduit which supplies the fibrous material into the interior of the chamber. The highly moisture-absorbent material and fibrous material are intermixed within the feed conduit to completely intersperse the moisture-absorbent material throughout the fibers prior to introduction into the forming chamber. This produces a non-woven pad atop the conveyor within the chamber in which the moisture-absorbent material is present throughout the entire thickness, width and length of the non-woven pad.
One problem with the above-described method is the loss of moisture-absorbent material through the perforated conveyor in the forming chamber. As the fibers and moisture-absorbent material mixture is drawn onto the perforated conveyor to form the non-woven pad, moisture-absorbent material at the lower portion of the non-woven pad is drawn through the conveyor into a filter-reclamation system. Some of the moisture-absorbent material is lost. Additionally, the moisture-absorbent material is difficult to contain even with the filtering system, and environmental contamination can result.
Another problem with this method is that the moisture-absorbent material is distributed throughout the non-woven pad across its entire length, width and thickness. This produces substantial waste because in subsequent forming operations, the non-woven pad is cut to the desired length of the hygienic article. In addition, the application of moisture-absorbent material across the entire width of the non-woven pad may be unnecessary for some types of hygienic articles, particularly disposable diapers where the leg holes are cut at the edges of the layer.
Another method of combining moisture-absorbent material with the non-woven pads employed in hygienic articles involves the application of moisture-absorbent material onto the top surface of the non-woven pad downstream from the leveling or scarfing rollers and outside of the forming chamber. Accordingly, a non-woven pad is produced having moisture-absorbent material concentrated on its top surface with substantially no moisture-absorbent material within the remaining thickness of the pad.
One disadvantage of this second method relates to the handling and loss of the moisture-absorbent material applied to the pad. Because the application of moisture-absorbent material takes place outside of the forming chamber in this method, a relatively expensive collection system is required to capture the oversprayed moisture-absorbent material and prevent it from escaping to the environment. Migration of the moisture-absorbent material out of the pad is a particular problem in this method because all of the powder is concentrated at its top surface. In particulate or granular form, the moisture-absorbent material can be readily dislodged from the type of pads which are not sealed at the ends.
Another problem with this second method of incorporating the highly moisture-absorbent material into the non-woven pad is that the material is concentrated on top of the pad. This causes so-called "gel blockage" wherein the moisture-absorbent material at the top of the non-woven pad become saturated with fluid and prevents the wicking or transfer of moisture to the fibrous material in the remaining portion of the pad. As a result, the fluid is retained at the surface of the pad in contact with the wearer of the hygienic article causing discomfort. Additionally, only a relatively small portion of the entire thickness of the pad is effectively employed to absorb the moisture.
These problems have been addressed in U.S. patent application Ser. No. 07/348,149, filed May 2, 1989, and entitled "Apparatus For Depositing Moisture-Absorbent Material In A Substrate", which is commonly owned by the assignee of the instant application. The apparatus disclosed in patent application Ser. No. 07/348,149 employs essentially the same type of forming chamber disclosed in U.S. Pat. No. 3,984,272, but is effective to intermix highly moisture-absorbent material with fibrous material in a predetermined portion of the thickness of the pad and, if desired, at longitudinally spaced locations therealong. A dispenser, such as a spray gun, is located at least partially within the forming chamber at a predetermined location with respect to the upwardly sloping top surface of the non-woven pad of fibrous material being drawn onto the conveyor. The dispenser is effective to intermix highly moisture-absorbent material with a portion of the fibrous material being drawn onto the conveyor at such predetermined location. This forms a layer of intermixed fibrous material and particulate material within a predetermined portion of the thickness of the pad while maintaining another portion of the thickness of the non-woven pad substantially free of particulate material.
Expressed in other terms, the apparatus disclosed in Ser. No. 07/348,149 forms a non-woven pad in which fibrous material is drawn onto the conveyor beginning at the inlet to the forming chamber, and such fibrous material begins to build up on the conveyor between the inlet and the location of the spray gun for the highly moisture-absorbent material. This forms a bottom layer of the non-woven pad resting directly on the conveyor which increases in thickness in the direction of movement of the conveyor through the chamber. The spray gun intermixes a second material, e.g., highly moisture-absorbent particulate material, with the fibrous material being drawn onto the conveyor at the predetermined location where the spray gun is positioned at least partially within the forming chamber. This intermixture of fibrous material and particulate material forms an intermediate layer which is drawn onto the bottom layer of fibrous material already accumulated on the conveyor upstream from the location of the spray gun. As the conveyor continues moving toward the outlet of the forming chamber, additional fibrous material is drawn toward the conveyor forming a top layer of fibrous material over the intermediate layer of intermixed fibrous material and moisture-absorbent material. As a result, a non-woven pad is formed having a bottom layer of fibrous material, an intermediate layer of intermixed fibrous material and particulate material and a top layer of fibrous material wherein the top and bottom layers of fibrous material are substantially free of moisture-absorbent material.
The apparatus described above which is the subject of Ser. No. 07/348,149 substantially improves upon the methods described above in a number of respects. The problem of gel blockage is substantially eliminated because the highly absorbent material is concentrated preferably at the core or center portion of the non-woven pad as opposed to near its upper surface. The loss of the particulate moisture-absorbent material is reduced and problems of overspray of such material are substantially eliminated. Additionally, by locating the moisture-absorbent material at selected areas within the thickness and length of the pad, a substantial savings in the amount of moisture-absorbent material required is achieved.
Despite the improvements obtained by the apparatus disclosed in Ser. No. 07/348,149, certain limitations have been observed in the manufacture of non-woven pads for some types of hygienic articles. For example, even though the highly moisture-absorbent, particulate material is concentrated within the center layer of the non-woven core using such method and apparatus, migration of the particulate material through the surrounding fibrous material layers can still occur. Given the further steps required in the process for forming the completed hygienic article, and the packaging and shipment of such hygienic articles, migration of the highly absorbent particulate material into the surrounding fibrous material or entirely through the non-woven pad is a possibility.
Additionally, it is desirable to increase the structural integrity of the non-woven pad, and particularly that portion of the thickness of the pad containing the highly moisture-absorbent material. It has been found that when the non-woven pad becomes wet, with such wetness being concentrated in the area of the highly moisture-absorbent material, the integrity and stability of the center of the pad is reduced and can present problems of tearing.